1. Field of the Invention
The present invention relates to an infrared imaging device and an infrared imaging system using such an infrared imaging device, and more particularly to an infrared imaging device which includes a detector comprising a thermocouple, a bolometer, or the like and a scanner comprising a charge-coupled device (CCD) or the like, and an infrared imaging system for outputting an image signal which is detected by such an infrared imaging device.
2. Description of the Prior Art
As shown in FIG. 1(a) of the accompanying drawings, a conventional thermal infrared imaging device includes both a detector comprising a thermocouple composed of a P-type polysilicon layer 208, a hot contact 209, and an N-type polysilicon layer 210, and a scanner comprising a CCD composed of an N-type well 202 and a CCD electrode 203 and a readout gate 204 and a source 205 for supplying a current to the CCD in response to a signal from the detector. The scanner is fabricated in a P-type semiconductor substrate 201 with an oxide film 206 deposited thereon by chemical vapor deposition (CVD), the oxide film 206 having a cavity 211 defined therein. The thermocouple is deposited on the oxide film 206 across the cavity 211. The cavity 211 is produced by forming a polysilicon layer (sacrificial layer) in the oxide film 206 where the cavity 211 is to be defined, and then etching away the polysilicon layer. The detector and the scanner are arranged in the vertical columns and horizontal rows of a two-dimensional pattern as an infrared imaging device as shown in FIG. 1(b).
The infrared imaging device shown in FIGS. 1(a) and 1(b) operates as follows: A certain bias voltage V.sub.B is applied to a bias voltage terminal 213 to operate readout transistors 215, which are each composed of a readout gate 204 and the source 205, in a suitable fashion. Infrared radiation applied from above in FIG. 1(a) is absorbed by a pellet surface, making the temperature of a thin-film area, i.e., the hot contact 209, above the cavity 211 higher than the temperature of the other area. A bias voltage line 207 and a contact 212 correspond to a cold contact of the thermocouple, and are positioned outside the thin film. Therefore, any heat applied to the bias voltage line 207 and the contact 212 is transferred to the substrate, resulting in substantially no temperature rise at the bias voltage line 207 and the contact 212.
Because of the difference between the temperatures at the hot and cold contacts, an electromotive force is generated due to the Seebeck effect, and added to the bias voltage V.sub.B, which is then applied to the readout gate 204 of the readout transistor 215. The voltage applied to the readout gate 204 modulates an electron flow from the source 205 to the N-type well 202 corresponding to a vertical CCD 216, which stores electric charges for a certain period of time. The electric charges stored in the vertical CCD 216 of each pixel as shown in FIG. 1(b) are successively transferred to horizontal CCDs 219 by the charge transfer operation of the CCDs, and then supplied through an on-chip amplifier 218 to an output terminal 217. The output terminal 217 supplies an output signal representative of the amount of infrared radiation incident on each pixel for thereby producing a two-dimensional infrared image.
An infrared imaging system which employs the infrared imaging device of the above structure includes a CCD driver for outputting drive signals to enable the CCDs to transfer the stored electric charges, and an amplifier for amplifying the signal supplied from the output terminal 217.
For increasing the sensitivity of the conventional infrared imaging device, it is necessary to reduce the thickness of the thin-film area, i.e., diaphragm, over the cavity 211 for thereby prevent the heat from being drained. The thickness and width of the P-type polysilicon layer 208 and the N-type polysilicon layer 210 have to be reduced because their thermal conductivity is relatively high. If the oxide film 206 has a small film thickness, it tends to develop pinholes, potentially causing the P- and N-type polysilicon layers 208, 210 to break when the polysilicon layer is etched away to produce the cavity 211, and also causing some diaphragms to deform and break owing to different fabrication process conditions.
The breakage of the thermocouple of even one pixel opens the gate of the corresponding readout transistor 215, allowing a number of electrons to flow into the vertical CCD and then to overflow successively into surrounding pixels until the infrared imaging device stops functioning.
Therefore, the breakage of the thermocouple of a single pixel is fatal because it results in the failure of all pixels. Such a phenomenon manifests itself when efforts are made to increase the sensitivity of the infrared imaging device, and hence is the greatest obstacle to increasing the sensitivity of the infrared imaging device.